Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: David on 13/12/2008 19:27:07

Title: Where does cement come from?
Post by: David on 13/12/2008 19:27:07
David asked the Naked Scientists:

The lime that was used in buildings, as a sort of cement or part of the mix, does this come from lime stone.  How is it made and is dangerous?

What do you think?
Title: Where does cement come from?
Post by: Atomic-S on 19/12/2008 07:02:30
I believe lime comes from limestone, and is derived from it by roasting. The reaction would be CaCO3 --> CaO + CO2.  The resulting CaO is then mixed with H2O to give Ca(OH)2, common lime. Common lime is strongly basic, and as such will tend to corrode living tissues, which is why you should not handle it a lot.
Title: Where does cement come from?
Post by: chris on 19/12/2008 08:43:28
So why is cement "sticky" and how does it glue bricks together?
Title: Where does cement come from?
Post by: lightarrow on 22/12/2008 14:43:20
Quote from: chris on 19/12/2008 08:43:28
So why is cement "sticky" and how does it glue bricks together?
It's "sticky" because it's made of a very fine powder and of chemicals (e.g. calcium aluminates, sulphate and silicates) wich forms strong hydrogen bonds with water.
It glues bricks together because, with time, calcium aluminates becomes hydrated with water (first phase, quick) and then calcium silicates becomes hydrated with water (second phase, hardening, slow) forming a net of chrystals with all the bulk, making it very rigid, with a structure similar to that of a rock.

Cement is made, essentially, by fine powdering and homogenizing clay + limestone and then heating this powder at 1400°C to eliminate CO2 and to form calcium aluminates and silicates.
Title: Where does cement come from?
Post by: Bored chemist on 22/12/2008 15:08:10
There's more than one sort of cement.
The simple lime mortar  used for old buildings was made from slaked lime  (Ca(OH)2) and sand.
This hardens slowly on exposure to air because it reacts with CO2 to form calcite.

The "portland cement" made from lime and clay hardens because of a different set of reactions (as Lightarrow says). Because it doesn't require CO2 to set it will still work under water.

The whole story is rather complicated; here's some useful data on it.
http://en.wikipedia.org/wiki/Cement
Title: Where does cement come from?
Post by: Pumblechook on 22/12/2008 19:18:23
B & Q ?
Title: Where does cement come from?
Post by: lyner on 22/12/2008 19:55:23
Pricey there - even on Wednesdays.